Early detection of breast tumors is considered essential in the treatment of breast cancer. The project undertakes to create a high sensitivity PET breast imaging system capable of detecting and locating concentrations of positron labeled radiopharmaceuticals in active and, potentially, in incipient breast lesions.The development of this instrument is in parallel with a DNM-NCI project to develop radiotracers with high specificity for breast tumors. The device consists of two opposed slabs of the scintillator BGO coupled to two position-sensitive photomultiplier tubes operated in time coincidence. The breast is imagined to be compressed between two detectors to increase detection sensitivity beyond that attainable with conventional PET ring tomographs. The two detectors will be mounted on an X-ray mammography unit in such a way as to obtain PET images of the breast in spatial registration with X-ray mammograms in the same imaging session. Preliminary (paper) design work for this system has begun and test bed studies defined that will permit competing technical approaches to this system to be evaluated.